From Wikipedia, the free encyclopedia
Skype (IPA pronunciation: /skʌɪp/, rhymes with type) is a peer-to-peer Voice over IP (VoIP) network founded by the entrepreneurs Niklas Zennström and Janus Friis, also founders of the file sharing application Kazaa. It competes against existing open VoIP protocols such as SIP, IAX, and H.323. The Skype Group, acquired by eBay in October 2005, is headquartered in Luxembourg, with offices in London, Tallinn and Prague.
Skype has experienced rapid growth in both popular usage and software development since launch, both of its free and its paid services. The Skype communications system is notable for its broad range of features, including free voice and video conferencing, and its ability to use peer to peer (decentralized) technology to overcome common firewall and NAT (Network address translation) problems.
System and software
The main difference between Skype and other VoIP clients is that Skype operates on a peer-to-peer model, rather than the more traditional server-client model. The Skype user directory is entirely decentralised and distributed among the nodes in the network, which means the network can scale very easily to large sizes (currently just over 100 million users) without a complex and costly centralised infrastructure.
Skype also routes calls through other Skype peers on the network to ease the traversal of Symmetric NATs and firewalls. This, however, puts an extra burden on those who connect to the Internet without NAT, as their computers and network bandwidth may be used to route the calls of other users.
The Skype client's application programming interface (API) opens the network to software developers. The Skype API allows other programs to use the Skype network to get "white pages" information and manage calls.
The Skype code is closed source, and the protocol is not standardized. The Windows user interface was developed in Pascal using Delphi, the Linux version is written in C++ with Qt, and the Mac OS X version is written in Objective-C with Cocoa. Parts of the client use Internet Direct (Indy), an open source socket communication library.
Critics point out that secure communication is not a feature:
- Skype reportedly uses openly available, strong encryption algorithms.
- The user is not involved in the encryption process and therefore does not have to deal with the issues of Public key infrastructure.
The Skype code is proprietary and closed source, and will not become Open-source software, according to a quotation:
..we don't have the time right now."
—Niklas Zennstrom, co-founder of Skype, responding to the Skype security model
The book from Que Publishing, Skype: The Definitive Guide points out partly:
- Skype can utilise other users' bandwidth. (Although this is allowed for in the EULA, there is no way to tell how much bandwidth is being used in this manner). There are some 20,000 supernodes out of many millions of users logged on. Skype Guide for network administrators  claims that supernodes carry only control traffic up to 5 kbytes/s and relays may carry other user data traffic up to 10 kbytes/s (for one video call). A relay should not normally handle more than one "relayed connection".
- Skype's file-transfer function does not contain any programmatic interfaces to antivirus products, although Skype claims to have tested its product against antivirus "Shield" products.
- The lack of clarity as to content means that firewalls and systems administrators cannot be sure what Skype is doing. (The combination of an invited and a reverse-engineered study taken together suggest Skype is not doing anything hostile.) Firewall rules for Ip tables were given to block Skype for corporates.
- The actual communication of any given Skype conversation is reported to appear relatively secure; both cryptographic analyses concluded that Skype had made good use of modern encryption techniques and had coded the actual encryption algorithms correctly within the software.
Skype accesses the hard disk several times per minute. This can be verified by observing the HDD's activity LED, or by using a file access monitor such as FileMon. With regard to internet bandwidth, certain users are selected by software to act as "supernodes". Under certain conditions, Skype is reportedly willing to accept thousands of connections, but is stated to limit itself to 40Kb/s upload and download.
Confidentiality of data
Skype claims that the proprietary session establishment protocol is efficient and prevents both man-in-the-middle and replay attacks. The software is not self-certifying which means it needs to connect and login to a centralized Skype server to certify each user's public key.
Skype currently permits multiple concurrent logins: if an attacker is able to obtain a user's login password, the attacker could login as that user, and change their status to "Hidden". Thereafter, any chat sessions involving the real user are possibly copied to the hacker's "ghost" account. Provided a user keeps his/her password secure, this is not of concern.
Authenticity of user identity
Skype provides an uncontrolled registration system for users: registration requires no proof (in means of state-issued ID card) of the identity of the user. This works two ways: you can use the system safely without revealing your real-life identity to other users of the system, but on the other hand you have no guarantees that the person you communicate with is the one they say they are in real life. The downside of this is that it is easy to use the personal name (but not identity) of a trusted person as a Skype nickname and trick a naive user into revealing information or executing a program sent to them.
It should be noted that this behavior is common to all digitally provided services: the exception is certificates from trusted certificate authorities with all the known drawbacks
- For detailed changelog see Skype changelog.
Versions now exist for Microsoft Windows (2000, XP and CE (Pocket PC)), Mac OS X and GNU/Linux. The Linux version runs on FreeBSD through its Linux binary compatibility; the Fedora Core version works fine, provided the user switches on the microphone in the GNOME sound settings. Symbian version is currently under development. 
Usage and Traffic
It was reported that eight million concurrent Skype users were online as of November 8, 2006. Gender information is inconclusive so far. More than half of all users declined to state their sex.
Although the volume of international traffic routed via Skype is significant, the quantity is still small when compared to a global switched and VoIP traffic base of 264 billion minutes. Computer-to-computer traffic between Skype users in 2005 was equivalent to 2.9% of international carrier traffic in 2005 and approximately 4.4% of total international traffic in 2006.
Skype incorporates some features which obfuscate its traffic, but it is not specifically designed to thwart traffic analysis and therefore does not provide anonymous communication. Some researchers have also been able to watermark the traffic so that it is identifiable even after passing it through an anonymizing network .
SkypeOut allows Skype users to call traditional telephone numbers, including mobile telephones, for a fee. This fee is as low as USD$0.024 per minute for most developed countries, and as high as USD$2.142 per minute for calls to the dependency of Diego Garcia. After 180 days of not making a SkypeOut call the Skype balance expires. This policy makes Skypeout a poor service for infrequent users as they will often lose their entire account balance.
Until December 31 2006, SkypeOut calls originated within the USA and Canada to destinations within USA and Canada are free of charge. The standard SkypeOut rate will be charged starting in 2007 , although an unlimited usage plan for North American calls is available. 
SkypeOut calls to toll free numbers in France (+33 800, +33 805, +33 809) , Poland: (+48 800) , UK: (+44 500, +44 800, +44 808 ) and the USA and Canada: (+1 800, +1 866, +1 877, +1 888 ) are free for all Skype users, even if they do not have the SkypeOut service. . SkypeOut calls to toll-free numbers in Belgium (+32 800) do not work.
On 19 December 2006, Skype accounced that there will be a new pricing structure in 2007. Details on the new scheme are scheduled to be released 18 January 2007. The press release is vague about the new scheme, but it does reveal that there may be a new connection fee.
SkypeIn allows Skype users to receive calls on their computers dialed by regular phone subscribers to regular phone numbers. Permits users to subscribe to numbers in Australia, Brazil, Denmark, Estonia, Finland, France, Germany, Hong Kong, Japan, Poland, Sweden, Switzerland, UK and USA.
For example, a user in San Francisco could create a local telephone number in Helsinki. Callers from Helsinki would pay only local rates to call that number.
Skype Voicemail was released on March 10, 2005. This service allows callers to leave voice-mail messages for Skype users who are indisposed.
Skype Voicemail has experienced numerous problems over the past year and users complain that many voice mail calls are never received. Additionally, the SkypeIn service occasionally fails to record certain incoming calls on the program's history page. These problems have not been completely resolved.
Skype supports group text chat with an interface similar to IRC with 100 People.
The Macintosh version uses the same message view style format as Adium, though with a different filename extension. Message view styles made for Adium can be installed for Skype, and they do not even need to be renamed. There are a couple of cosmetic bugs, but ignoring those, Adium styles work without modification. This feature is not present in the Windows, Linux, and Pocket PC versions of Skype.
Skype video calling
On Windows XP (Windows 2000 users require DirectX 9.0 for video calls) and Mac, Skype 2.0 (and above) supports videoconferencing, making Skype one of the few cross-platform video conferencing solutions between Windows and Mac. Skype only supports one-to-one video chat.
Skypecasts was released on May 3, 2006. Skypecasts are live, moderated conversations allowing groups of up to 100 people to converse, moderated by the "host" who is able to mute, eject or pass the virtual microphone to participants when they wish to speak. Skypecasts do not support chat windows to share text information (such as URLs) with participants.
One of features of Skype 2.5 (and above) is the ability to send SMS messages to mobile phone numbers (a feature commonly used in other IM software such as ICQ, AOL Instant Messenger, Yahoo Messenger and Windows Live Messenger).
Skype web toolbar
The Skype Web Toolbar recognizes phone numbers and Skype Skype. Currently only available for Internet Explorer and Mozilla Firefox on Windows.
Skype Zones Beta is software powered by Boingo that provides access to Skype through Skype-friendly hot spots.
Skype history logs
A log file is created for each contact on a user's contact list. Log files are stored locally, meaning they are not available if a user switches computers. By default, the option to log conversations is disabled, but can be enabled in the tools / privacy panel.
Recently a number of manufactures have launched hardware devices compatible with Skype. These are usually either tethered to a PC, or have a built-in Wi-Fi client to allow calling from Wi-Fi hotspots like the Netgear SPH101 Skype Wi-Fi Phone.
Skype Beta version 3.0 provides additional tools under "Do More" Submenu. Tools added include the 'Pamela Voice' recorder for recording voice calls, plus many games.
Skype has been criticised over its use of a proprietary protocol, instead of an open standard like H.323, IAX, or SIP, since this makes it impossible for other providers to interact with the Skype network. There are of course clear business reasons for this, since it helps protect the SkypeOut revenue stream from competition.
A design limitation of Skype is that, if given access to an unrestricted network connection, Skype clients can become supernodes. These supernodes hold together the peer-peer network and provide data routing for those behind restrictive firewalls. Unfortunately, these supernodes can generate a significant amount of bandwidth. For this reason some network providers, such as universities, have banned Skype.
A third party paper analyzing the security and methodology of Skype was presented at Black Hat Europe 2006. It analyzed Skype and made these observations:
- Heavy use of anti debugging techniques (used to deter development of alternative clients, hacking tools)
- Significant use of obfuscated code (slows reverse engineering, less description of what program code does internal to the executable file)
- Keeps chatting on the network, even when idle (even for non-supernodes. may be used for NAT traversal)
- Blind trust in anything else speaking Skype
- Ability to build a parallel Skype network
- Lack of privacy (Skype has the keys to decrypt sessions)
- Heap overflow in Skype
- Skype makes it hard to enforce a (corporate) security policy
- "No way to know if there is/will be a backdoor"
Another criticism of Skype has been content filtering. See: Internet censorship in the People's Republic of China.
While available for most operating systems, there is no Skype version for the Palm OS which is widely used in mobile devices like the Treo 700p.
Skype has been heavily criticized in the Linux community for bugs and delays in the Linux version .
Legal and political aspects
Skype faces challenges from two main legal and political directions - challenges to its intellectual property, and political concerns by governments who wish to exert more formal control over aspects of their telecommunications systems.
Skype's technology is proprietary and closed to outside review. It is unknown to what extent it can potentially intrude upon other parties' patents and copyrights. It is not unreasonable, therefore, to expect legal challenges from third parties concerning Intellectual Property issues.
Skype also supply Skype-out phonelines without requiring proof of address, which is illegal in some countries.
In January, 2006, StreamCast Networks filed a complaint in U.S. District Court in Los Angeles, accusing Skype of stealing its peer-to-peer technology. The $4.1 billion lawsuit did not initially name eBay, Skype's parent company; however, the lawsuit was amended in a filing with Federal Court in the Central District of California on May 22, 2006, to include eBay and 21 other parties as defendants.
Streamcast seeks a worldwide injunction on the sale and marketing of eBay's Skype Internet voice communication products, as well as billions of dollars in unspecified damages.
On June 1, 2006, Net2Phone (the Internet telephone unit of IDT Corp.) filed a lawsuit against eBay and Skype accusing the unit of infringing U.S. Patent 6,108,704, which was granted in 2000.
For a brief period, SkypeOut was blocked in some regions of mainland China (notably Shenzhen) by the operator China Telecom for undisclosed reasons; it has been speculated that this may relate to SkypeOut's ability to take lucrative international and long-distance business away from the People's Republic of China's state-controlled telecommunications companies.
Skype is one of many companies (others include AOL, Google, Microsoft, Yahoo, Cisco) which have cooperated with the Chinese government in implementing a system of Internet censorship in mainland China. Critics of such policies argue that it is wrong for companies to assist in such policies, which might allow them to profit from censorship and restrictions on freedom of the press and freedom of speech. Human rights advocates such as Human Rights Watch and media groups such as Reporters Without Borders state that in their view, if companies stopped contributing to the authorities' censorship efforts the government could be forced to change.
Niklas Zennstrom, chief executive to Skype, told reporters that its joint venture partner in China is operating in compliance with domestic law. "Tom Online had implemented a text filter, which is what everyone else in that market is doing," said Mr Zennstrom. "Those are the regulations." "I may like or not like the laws and regulations to operate businesses in the UK or Germany or the US, but if I do business there I choose to comply with those laws and regulations. I can try to lobby to change them, but I need to comply with them. China in that way is not different."
In September 2005, the French Ministry of Research, acting on advice from the general secretariat of national defence, issued an official disapproval of the use of Skype in public research and higher education; some services are interpreting this decision as an outright ban. The exact reasons for the decision were not given, but speculatively may relate to issues noted earlier, relating to inability to monitor the nature of information being communicated, possible extreme resource usage, or unknown potential actions of the software.
In December 2006, the Government of India announced they are preparing a crackdown on VoIP services, citing security risks and loss of revenue. The clampdown is targeted at outsourcers and other Indian IT businesses that use foreign owned VoIP services, such as Skype and Yahoo!, to cut their phone bills and evade the six percent revenue share and 12 percent tax imposed on local services by the government. According to the The Times of India, companies must reveal the names of licensed service providers they purchase bandwidth and internet telephony minutes from. Companies will also have to undertake that they will not use the services of unlicensed internet service providers.
Skype group (corporate)
On October 14, 2005, eBay acquired the company for €1.9 billion in cash and stock, plus an additional €1.5 billion in rewards (earn out) if goals are met by 2008.
Competition and alternatives
Open source alternatives
- Ekiga: A free application that supports both H.323, SIP, audio and video. Ekiga was formerly known as GnomeMeeting. So far works only with various Linux based systems. No version for Microsoft Windows has been released yet, but there is a working snapshot available.
- Kiax: VoIP application based on IAX.
- PSI: The current Beta version has protocol support for Google Talk.
- Switchboard: Free VoIP applet which works from within a web browser. Works on Windows, Mac, Linux, and any other Java enabled platform. No installation necessary.
- Tapioca: Includes support for Google Talk.
- WengoPhone: A free VoIP application based on SIP open standard.
Closed source alternatives
- amiciPhone: A secure peer-to-peer VoIP application
- Google Talk: A popular service provided by Google
- Gizmo Project: A closed source VoIP application based on SIP open standard and uses SRTP between clients. Now offering free landline/cell calls to over 60 countries
- iCall: A closed source free VoIP application based on SIP open standard and providing free PC to Phone calling in the US and Canada.
- Jajah: Alternative where no headset, no download, no installation and no broadband connection is necessary. A VoIP call gets activated between two normal phones.
- Secure Shuttle Transport (SST): Free encryption and secure messaging software including VoIP and video. Works on PCs running Windows 98 or higher.
- Raketu: A VoIP service that combines communication, information, and entertainment. Its integrated multi-messenger allows communication with contacts from AOL Instant Messenger, ICQ, MSN Messenger, Google Talk, Yahoo Messenger and Skype.
- SightSpeed: Free video and voice calling service supporting Mac & Windows. Also allows phone out and in calling.
- Parlino: A VoIP network based on open standard SIP-protocols, launched by Parlino S.A.
- Vbuzzer: A VoIP softphone and service as well as an active advocator of SIP open standard
- VoipBuster: A VoIP application offering 300 minutes per week of free calls to landlines in many countries, including the EU, USA, Australia, etc.
- VoipStunt: A VoIP application offering 300 minutes per week of free calls to landlines in many countries, including the EU, USA, Australia, etc.
- Zfone: A solution of Phil Zimmermann (inventor of PGP) to encrypt VoIP (SIP) sessions, protocol published as IETF draft. 
- TipicIM: A free VoIP application, Videocalling based on XMPP/Jabber and Speex audio codec support
- [ClosedTalk]®: A secure VoIP software free from CE-Infosys for Business/Personal use. Works on PCs running Windows 2000/XP. [ClosedTalk]" exposes �man in the middle� attacks by displaying a short security message on both caller screens for comparison.
- BT Communicator: A VoIP service from British Telecom (BT plc.)
- Skype Protocol
- Presence information
- Voice over IP
- Comparison of VoIP software
- Instant messaging
- Comparison of instant messaging clients
- Comparison of instant messaging protocols
- Secure communication
- Skype Journal: An independent online magazine about Skype
- Nuvvo eLearning Service: an on-demand service with SkypeWeb Presence integration
- ^ Jaanus Kase. Skype is expanding engineering to Prague. Skype Blogs. Retrieved on 2006-12-05.
- ^ Interview with Jaanus Kase from Skype. KDE News. Retrieved on 2006-06-13.
- ^ Skype Privacy FAQ. Skype. Retrieved on 2006-12-05.
- ^ "VoIP suffers identity crisis", The Register, June 15, 2004.
- ^ Harry Max. Skype: The Definitive Guide. Retrieved on 2006-08-22.
- ^ FileMon for Windows. Sysinternals. Retrieved on 2006-06-17.
- ^ Bruno Giussani. Swiss magazine digs deeper in social blog. Lunch over IP. Retrieved on 2006-06-17.
- ^ Fear of a Skype Planet. Paul Kedrosky. Retrieved on 2006-06-17.
- ^ Skype supernodes sap bandwidth. Computerworld. IDG. Retrieved on 2006-06-17.
- ^ Jack McCarthy. China bans Skype. InfoWorld. Retrieved on 2006-06-17.
- ^ eBay Completes Acquisition of Skype. eBay. Retrieved on 2006-06-17.
- ^ Skype Launches Next Generation Free Internet and Video Calling for Everyone. Skype. Retrieved on 2006-06-17.
- ^ Skype to Announce Disruptive Pricing Strategy for SkypeOut Retrieved: December 19, 2006
- ^ 
- ^ 
- ^ Jaanus. Eight million online. Skype Blogs. Retrieved on 2006-11-08.
- ^ International carriers' traffic grows despite Skype popularity. TeleGeography Report and Database. Retrieved on 2006-12-07.
- ^ Skype End of Service Announcement. skype.com. Retrieved on 2006-12-17.
- ^ SkypeOut Unlimited Calling. skype.com. Retrieved on 2006-12-17.
- ^ Adding chat styles to Skype. blodspot.com. Retrieved on 2006-06-13.
- ^ Adium message styles and Skype (1 updates). Soeren Says. Retrieved on 2006-06-13.
- ^ Philippe BIONDI and Fabrice DESCLAUX. Silver Needle in the Skype. blackhat. Retrieved on 2006-03-02.
- ^ Pallavi Gogoi. "Skype Under Attack", Business Week, McGraw Hill, June 6, 2006.
- ^ Skype uses peer-pressure defense to explain China text censorship. The Register. Retrieved on 2006-06-18.
- ^ eBay Completes Acquisition of Skype. Skype. Retrieved on 2006-06-12.
- ^ "eBay to buy Skype in $2.6bn deal", BBC, September 12, 2005.
- Official Skype website
- Scientific Research on Skype (Reverese Engineering, etc.)
- White paper on Skype, focus on supernodes
- Skype Security Evaluation by Tom Berson (Anagram Laboratories)
- Graze the page of Skype
- Paper discussing Skype's internals from a security perspective (Black Hat Europe 2006) - Philippe Biondi & Fabrice Desclaux
- Skype, Zennstrom, Friis Et Al Sued for RICO Violations
- Kodak Photo Voice for Skype
- Skype to Charge for Unlimited Phone Call Plan
- Detecting Skype and P2P traffic - A general identification method